Since I had the twins i have had a strained relationship with my MIL. It is obvious that she feels jealous of me and I secretly feels she hates me as she is always sending cutting texts and emails.
She recently had heart trouble and I would send similar emails/texts to her wishing her well as my husband was sending. He would get back lovely kisses and I would get things like: "of course I'm not alright" and "don't you realise how serious this is".
The Mummy thing. The fact you son was upset about it - and that he felt
While not going as far as some posters, I think there is cause for concern here.
if your husband will not stand up for you it is very upsetting, both the original problem and the fact that well - a woman wants to know that she comes first with her DP, not the man's mother! Being dismissed or unsupported makes everything, including one's own reactions, worse.
I think that this needs to be talked about with your husband becuase you and he need to stand firm together and he needs to put you first.
It is possible that she is pretty toxic - it's also possible that it's not as bad as all that. But the fact that she is responding differently to pretty similar emails from you and from your DP, is concerning. It does look like there's possibly some sort of power struggle here and it's one you need to win.
Is it possible to talk to her direct and say look, please don't let him call you Mummy again? and depending on her response, then either things will be improved or else you can decide to simply let your boys go up there less, or for a shorter time. it will mean fights with her and perhaps your DP, but you have a right to be spoken to decently by her as by anyone else and her attitude atm is not pleasant.